1. Field of the Invention
The present invention relates to a suction pad for attracting a work so as to enable the work to be fed to a desired position.
2. Description of the Related Art
There has heretofore been used a suction cup or pad which communicates with a vacuum suction source in order to feed, for example, a thin plate-shaped work to a predetermined position. This type of suction pad normally has a suction base and a suction skirt integrally provided in continuation with the base. In addition, the suction pad is mounted on an arm of a robot through a connector and used to feed a work, for example. At this time, when a tube extending from the vacuum suction source is connected to the base of the suction pad in such a way as to enable the vacuum suction source to suck or draw air inside the skirt, the work is attracted and held by the suction pad under reduced pressure of the skirt so as to be fed to a desired position.
The suction pad employed in the prior art will now be described with reference to FIGS. 1 and 2.
In order to feed a work W attracted and held by a suction pad 2, the distal end of a thin and flexible suction skirt 4 of the suction pad 2 is brought into contact with the work W. Then, the vacuum suction source is energized to suck or draw air inside the skirt 4 through a through hole 7 defined in a suction base 6 and an adapter 8 mounted on the base 6. As a consequence, the skirt 4 serves to attract and hold the work W from the distal end thereof under the reduced pressure of the skirt 4, thereby feeding the same to a desired position.
In this case, as the sectional form of the skirt is properly used either a circle or an ellipse according to the shape, weight, etc. of a work to be fed.
However, in the above prior art, when the suction pad 2 attracts and holds the work W, the end of the skirt 4 is brought into abutment against the work W. However, a contact area of the suction pad 2 with respect to the work W is increased or decreased according to the pressure, i.e., vacuum for sucking or drawing the air inside the skirt 4. When a vacuum, i.e., negative pressure inside the skirt 4 is increased in a state in which the suction pad 2 attracts and holds the work W as shown in FIG. 1 by way of example, the area of a contact surface 9 of the skirt 4 with respect to the work W is increased as shown in FIG. 2. Specifically, when the pressure for sucking or drawing the air inside the skirt 4 is increased or decreased at the time the suction pad 2 attracts the work W, the area of the contact surface 9 of the suction skirt 4 with respect to the work W changes. This shows that the attraction or suction force of the skirt with respect to the work is changed according to the change in the contact area.
Thus, when the vacuum for sucking the air inside the skirt is established to an initial condition, the theoretical value of the attraction or suction force is subjected to a complex calculation, and hence the accuracy of its calculation is low.
In the above-described suction pad, the position where the suction pad attracts and holds the work is displaced so as to move a part of the skirt away from the work. When the inside of the suction pad reaches the atmospheric pressure, the work is released from the skirt and therefore drops away from it.
When it is desired to feed a work having low rigidity, the work tends to drop. Consequently, space occurs between the work and the suction surface of the suction pad to produce a vacuum break, thereby releasing the work from the skirt while the work is being fed.